Age of Acquisition (AoA) refers to the age at which a concept is learned. Early-acquired words have an advantage over late-acquired words in processing accuracy and speed. Which stage of AoA playing its role in spoken word production remains controversial. The phonological completeness hypothesis assumes that AoA may have a phonological locus, while the semantic hypothesis assumes that AoA affects semantic processing (i.e., conceptual preparation, lexical selection). The network plasticity hypothesis assumes that AoA arises at multiple processing levels, in spoken word production.In a picture naming task, we used the event-related potential (ERP) technique to examine the loci of AoA effect in object and action pictures naming. Twenty-eight participants (9 males, mean range: 22.18, SD: 2.56) participated in this study. We selected a total of 188 words and their corresponding black and white line pictures, half of which were object pictures, and half were action pictures. Within each type of picture, half were early acquired, and half were late acquired. Therefore, the age of acquisition of picture names (early vs. late) and word type (noun vs. verb) served as within-participants variables. During the experiment, participants were asked to name each picture as accurately and quickly as possible.Behavioral data indicated a typical AoA effect in object pictures naming, showing that object pictures corresponding to early-acquired nouns were named faster than those corresponding to late-acquired ones. In contrast, action pictures corresponding to early-acquired verbs were named slower than those corresponding to late-acquired verbs. ERP data also showed distinct AoA effect patterns in object and action picture naming. For object picture naming, late-acquired nouns elicited a larger positivity than early-acquired nouns between 250~300 ms over left-prefrontal regions. In contrast, for action picture naming, early-acquired verbs evoked a larger positivity than late-acquired verbs within 200~250 ms, 300~400 ms and 450~600 ms time windows over the left hemisphere.We suggest that the AoA effect in object naming may originate in the lexical selection of spoken word production, supporting the semantic hypothesis. In contrast, the AoA effects in action naming may originate in multiple processes, such as lexical selection, phonological encoding and phonetic encoding, supporting the network plasticity hypothesis. The distinct AoA effects between the naming of object and action pictures probably relate to the distinct semantic networks that represent objects and actions. Therefore, the AoA effect in action picture naming is much more complicated than in object picture naming and needs further investigation.